1. Field of the Invention
The present invention relates to a heat-shrink tube for an electrical power cable, in particular a medium voltage power cable operating at voltages typically between 12 kV and 42 kV inclusive.
2. Summary of the Prior Art
Heat-shrink tubes for electrical power cables use heat-shrinkable polymeric technology to provide one or more heat recoverable sleeves having appropriate electrical characteristics which are shrunk into position around the ends of cables that have been electrically connected together. There are two main types of heat-shrink tubes currently in use for medium voltage (MV) electrical power cables.
A first type of heat-shrink tube is shown in FIGS. 1a and 1b and comprises a two-piece system that uses only heat-shrink materials to provide two heat recoverable sleeves that respectively provide insulation and an external conductive layer, which is required at the cable joint. An insulating-only sleeve 1 is installed first around the cable joint. A second, dual-layer sleeve 2 with a conductive outer layer 3 and an insulating inner layer 4 is then installed over a top of first sleeve 1 to provide a thicker insulating layer and the required conductive outer layer.
This heat-shrink tube is limited by the fact that it is not practical to manufacture sleeves comprising a thick wall of thermoplastic, heat-shrink material owing both to manufacturing difficulty, and to the problem posed in heating through the entire wall thickness of a thick tube sufficient to recover it, without over-heating the outer surface to the extent that damage occurs. This means that the maximum wall thickness of the insulating material is limited making it necessary to use multiple of the insulating-only sleeves 1 to create the required insulation thickness. Typically, two such sleeves are required for power cables operating at voltages up to around 24 kV. However, three or more of the insulating-only sleeves 1 are required if a higher voltage rating is required for the joint. The use of multiple insulating-only sleeves 1, however, causes its own problems not only because installation is prolonged but also because the increased number of interfaces between the multiple insulating-only sleeves 1 can lead to electrical problems as a result of air entrapment, contamination of the cable joint and the like.
A second type of heat-shrink tube is shown in FIG. 2 and comprises a single-piece, elastomeric insulating sleeve 5 in which a conducting heat-shrink outer layer 6 is used as a support mechanism for an elastomeric, insulating inner layer 7. The insulating inner layer 7 exerts an elastic force to shrink the insulating sleeve 5 but is prevented from doing so by the conductive, thermoplastic outer layer 6 that remains rigid until it is heated.
This second type of heat-shrink tube obviates the problems created by the use of multiple sleeves by replacing the inner thermoplastic insulating sleeve with the elastomeric insulating inner layer 7 that can recover without needing heat. This elastomeric insulating inner layer 7 is retained in an expanded form by the rigid conductive outer layer 6 that prevents recovery of the insulating sleeve 5 until the outer layer 6 is heated during installation. However, there are two main problems with this system. First, the elastomeric insulating inner layer 7 is slower to recover than the heat-shrink materials used in the first system. Second, because the elastomeric insulating inner layer 7 is not rigid, the only mechanism which prevents its recovery prior to installation is the rigid conductive outer layer 6. This means that the outer layer 6 is usually thicker than would otherwise be required for electrical reasons, thus adding materials and therefore cost to the product.